Thin film stress measurement by fiber optic strain gage

In this work, we present a new optical Fiber Bragg Grating (FBG) strain sensor which can be utilized to measure in situ the induced substrate strain caused by the film stress. The fiber optical sensor is interfaced to an analyzer to determine the wavelength shift of the reflected signal due to strain and temperature effects. The technique was used during the deposition of silicon dioxide and silicon carbide (SiC) thin films by RF magnetron sputtering, onto thin (38 μm) stainless steel substrates. The direct measurement of the wavelength shift Δ λ provides the strain value ε which can be used to find the residual stress σ present in the film/substrate interface. The preliminary results for SiC are in agreement with other works reported in the literature. The uncertainty provided by the measuring system is about 1 μm/m. Research is under way to improve the accuracy and study the possibility to develop a more compact device.